[109.01] Recent Results from the Masers in NGC4258: Detection of Nuclear Continuum Emission, and Progress in Deriving a Geometric Distance

VLBI observations of the water masers in the nucleus of the weakly
active Seyfert 2 galaxy NGC4258 reveal a nearly edge-on disk in almost
perfect Keplerian rotation. The disk dynamics provide compelling evidence
that the central mass is a massive blackhole. The disk is extremely
thin and appears to be slightly warped. This contribution presents two
recent results: the detection of 1.3 cm continuum emission in the
vicinity of the dynamical center of the maser disk, and progress in
determining a geometric distance to NGC4258.

22 GHz continuum emission has been detected in 3 of 4 VLBA+phased VLA
observations. These data, together with a program of VLA snapshots,
indicate that the continuum emission is approximately 100% variable on
timescales of weeks, and has an average flux density of 3 mJy. Most
of the emmision is located 0.5--1.0 mas (0.015--0.03 pc) north of the
dynamical center of the disk, along the projected disk rotation
axis. This axis is itself well-aligned with large
scale jets extending to kpc scales. The northern emission is unresolved
and has a brightness temperature of >10^7 K. We conclude that we have
detected synchrotron emission from the base of the
northern jet. The offset from the dynamical center is
consistent with that predicted for a mildy relativistic jet. The
southern continuum emission is also along the rotation axis, but is
consistently >3 times fainter. This may be due to thermal
absorption in the disk.

In principle, the masers can provide a purely geometrical distance
to the NGC4258 which is independent of all other distance
indicators. The distance is determined by observing accelerations and proper
motions of the systemic features as they rotate with the disk. We estimate
that distances accurate to 5% are achievable given current uncertainties
in the disk fitting process. Proper motions and accelerations have been
detected from the VLBI data, and they are consistent with the
expected motions in the disk. Our progress in deriving a geometrical distance
from these data will be discussed at the meeting.